Similar to photodiodes or imaging devices that convert optical signals into electrical signals, solar cells are an apparatus that displays an electrical output against an optical input and are an apparatus that displays a reverse response to that of electroluminescence (EL) devices that display an optical output against an electrical input. In recent years, such a solar cell has been being greatly watched as a clean energy source against the background of a fossil fuel depletion problem or a global warming problem, and research and development have been keenly carried out. Though silicon based solar cells using monocrystalline silicon, polycrystalline silicon, amorphous silicon or the like have been put into practical use so far, in view of the facts that the silicon based solar cells are expensive and that a shortage problem of raw material silicon or the like has become an issue, a demand for the development of a next-generation solar cell is increasing. Under such a background, an organic thin film solar cell which is inexpensive, low in toxicity and free from concern of a shortage of raw material greatly gets attention as a next-generation solar cell following the silicon based solar cell.
At the beginning, the research on the organic thin film solar cell was advanced on the basis of a single layer film using a merocyanine dye or the like. However, as a result of further research and development, it was found that a conversion efficiency (photoelectric conversion efficiency) from an optical input to an electrical output is enhanced by using a multilayered film having an “n-layer” that transports an electron and a “p-layer” that transports a hole, and since then, the multilayered film has become the main current. Materials used at the beginning of studying the multilayered film were copper phthalocyanine (CuPc) for the p-layer and a peryleneimide (e.g., PTCBI) for the n-layer, respectively. Thereafter, though it was found the photoelectric conversion efficiency is enhanced by inserting an “i-layer (mixed layer made of a p-material and an n-material)” between the p-layer and the n-layer to increase the lamination, the same materials were still used for the p-layer and the n-layer, respectively.
Thereafter, it was found that the photoelectric conversion efficiency is more enhanced by a stack cell configuration in which several layers of “p-layer/i-layer/n-layer” are repeatedly laminated. Materials used at that time were a phthalocyanine for the p-layer and a fullerene (C60) for the n-layer, respectively.
On the other hand, in organic thin film solar cells using a polymer, a research of a so-called bulk heterostructure in which a conductive polymer and a C60 derivative are used as a material of the p-layer and a material of the n-layer, respectively, and these materials are mixed and thermally treated to induce micro layer separation, thereby increasing a hetero-interface and enhancing the photoelectric conversion efficiency was chiefly carried out. Material systems used herein were chiefly poly-3-hexylthiophene (P3HT) as the material of the p-layer and a C60 derivative (PCBM) as the material of the n-layer, respectively.
In the light of the above, in the organic thin film solar cells, the materials of the respective layers have not developed so much from about the beginning, and phthalocyanine derivatives, peryleneimide derivatives and C60 derivatives are still used. In consequence, in order to increase the photoelectric conversion efficiency, the development of a new material as a replacement of these conventional materials is earnestly desired.
Now, in general, the operation process of an organic solar cell is composed of an elementary process including (1) light absorption and exciton formation, (2) exciton diffusion, (3) charge separation, (4) carrier transfer and (5) electromotive force generation, and there are generally not many organic materials displaying an absorption characteristic in agreement with a sunlight spectrum. Thus, in many cases, a high photoelectric conversion efficiency could not be achieved. For example, in organic EL devices, the development of which is energetically advanced in recent years, amine compounds which are excellent as a hole injection material as well as a hole transport material have been discovered. However, even when such an amine compound is used as the material of the p-layer for organic thin film solar cell, there is involved such a drawback that the absorption characteristic against the sunlight spectrum is insufficient, so that a sufficient photoelectric conversion efficiency is not obtained.
In general, it is known that in order to bear absorption in a visible light region on an organic compound, it would be good to expand a π-electron conjugated structure to make an absorption maximum wavelength long. However, when the conjugated system is overly expanded to make the molecular weight excessively large, there are encountered such a fault that not only the solubility in a solvent is lowered to make it difficult to achieve purification, but a sublimation temperature rises to make it impossible to achieve sublimation and purification, and so forth. Then, polyacenes have been developed as a material capable of efficiently making the absorption wavelength long while controlling the molecular weight to some extent (see Patent Documents 1 to 3).